Does anybody know where I can find a replacement gear set for the roto start back plate on the Genesis?

I use EBay.

Search .com as more is avalible in the States.
Delivery is only a few quid and arrives in a week, probably quicker than you local hobby shop.

My guess is that you'll have to buy the whole Roto backplate assembly .. 'bout $12 here. Also mount it so that the wand goes in at the lower right ( looking from the rear ). Friends that have mounted it straight up have had probs as the drive wand has to go in at an angle .. not good. Hope this helps you.